Any conventional vehicle is provided with a transmission system between an internal combustion engine and the driving wheels. Any conventional transmission system varies the driving force and speed of the wheels in accordance with widely variable driving conditions of the vehicle itself to allow the engine to fully exert its own performances. In conjunction with the transmission system, there are a variety of continuously variable transmission systems like those which are disclosed in official publications of Japanese Patent Laid-Open Nos. 57-186656 (1982), 59-43249 (1984), 59-77159 (1984), and 61-233256 (1986) for example. Any of these conventional continuously variable transmission systems transmits the driving force by either expanding or contracting the radius of rotation of a belt by either expanding or contracting the width of a belt channel formed between a stationary pulley member secured to the revolving shaft and a movable pulley member which is axially movably mounted to the revolving shaft so that the movable pulley member can come into contact with and move away from the stationary pulley shaft to vary the belt drive ratio.
The controlling of a clutch of a conventional continuously variable transmission system causes a switch to remain inactivated during the hold mode, while only the clutch engaged pressure is held at an objective clutch pressure, and thus the clutch-engaged condition (i.e., the creeping amount) remains constant. As a result, the control loop does not have the factor controlling the clutch engaging condition in response to the number of revolutions per unit of time of the engine. This causes the driver to feel a sense of incongruity, namely, hearing the sound of a fast running engine and noting a slow moving vehicle. In addition, since the clutch pressure sharply rises when entering into the start mode, certain shock may be generated when the driver causes the vehicle to begin moving.
Furthermore, since the clutch-engaged condition is not variable by the shift-lever position, engine braking cannot be applied while driving the vehicle at a slow speed despite the driver's intention. Furthermore, when the vehicle starts to move on a down slope, since the controller unit does not control the clutch-engaged condition, engine braking cannot be applied at all.
The primary object of the invention is to fully overcome those disadvantages mentioned above by providing a novel method of controlling a clutch of a continuously variable transmission system by implementing the provision of the following: a clutch-pressure controlling oil-pressure circuit which controls the clutch pressure in response to signals from a controller unit which controls clutch pressure by feeding actually-detected clutch pressure back to an objective clutch pressure; means for causing the controller unit to receive a variety of signals denoting the vehicle-driving conditions and output signals to the oil-pressure circuit when identifying the presence of a hold mode; and means for causing the oil-pressure circuit to properly control the clutch pressure to the predetermined level. As a result, the clutch-engaged condition can be varied by clutch pressure, thus significantly improving the engine-braking effect when the vehicle runs at a slow speed, and at the same time effectively minimizing the clutch-engaging shock.
To achieve the above object, a system for controlling the continuously variable transmission related to the invention controls transmission by either expanding or contracting the width of a belt channel between a stationary pulley member and an axially movable pulley member to either expand or contract the radius of rotation of a belt wound on these pulleys for eventually varying the belt ratio, in which the transmission control system embodying the invention is provided with the following: a controller unit which executes control by feeding actually-detected clutch pressure back to the objective clutch pressure; and an oil-pressure circuit which controls clutch pressure in response to signals from the controller unit. In addition, the clutch control method is comprised of the sequential steps of: delivering a variety of signals denoting the status of the drive vehicle to the controller unit; and causing the controller unit to deliver output signals to the oil-pressure circuit when the controller unit identifies the presence of a hold mode before eventually causing the oil-pressure circuit to control the clutch pressure to the predetermined level.